KIJIREPO-22. Ensures safe concurrent model deployment.

issue: https://jira.kiji.org/browse/KIJIREPO-22
review: https://review.kiji.org/r/1099/
reviewer: amit, milo, ajprax
KIJIREPO-26. Determine how model-repo artifact itself is packaged.

issue: https://jira.kiji.org/browse/KIJIREPO-26
review: https://review.kiji.org/r/1132
Merge from upstream